A nickel electrode consists of basically two parts, an electrochemically active material and an electrochemically inactive electrode substrate. The active material consists of various types and phases of nickel-oxide and/or nickel-hydroxide. The active material traditionally contains cobalt-oxide and/or cobalt-hydroxide as a performance enhancing additive.
The electrode substrate consists of a variety of materials and structures, including sintered nickel powder, nickel fibers, nickel foam or various forms of carbon and combinations of the aforementioned materials.
The active material undergoes reversible electrochemical reactions and imparts electrochemical energy storage properties to the electrode, and therefore the battery. The active material is amorphous and has no structural properties in and of itself. The electrode substrate provides mechanical support to the active material and also provides electrical conduction to the active material. The electrode substrate is electrochemically inert (other than corrosion), and provides no energy storage to the electrode. The electrode substrate is typically microporous and has a high surface area to maximize the active material/substrate interface. The active material typically exists within the micropores of the substrate.
A traditional nickel electrode swells in thickness primarily because of forces exerted on the metallic substrate by the active material. Nickel-hydroxide exists in multiple phases, degrees of hydration and oxidation states. In the generally accepted reaction scheme .alpha.-phase nickel-hydroxide (.alpha.-Ni(OH).sub.2), is deposited in the electrochemical process. This material ages to .beta.-phase nickel-hydroxide (.beta.-Ni(OH).sub.2) in aqueous potassium-hydroxide (KOH), by what is principally a dehydration process. The primary reaction pathway in a nickel electrode is the oxidation and reduction of .beta.-phase nickel-hydroxide between the charged (.beta.-NiOOH) and discharged (.beta.-Ni(OH).sub.2) states. If .beta.-NiOOH is charged further it can be converted to a .gamma.-phase nickel-hydroxide (.gamma.-NiOOH). The discharge of .gamma.-NiOOH yields .alpha.-Ni(OH).sub.2. The active material undergoes a number of structural, textural and density changes.
The structure of .alpha.-Ni(OH).sub.2 is described as turbostratic brucite-type layers with a random orientation along the c-axis. Water (H.sub.2 O), molecules and anions (primarily nitrate (NO.sub.3.sup.2-)) from the impregnation bath are dispersed in the interlamellar spaces. The material has a density of approximately 2.5 grams per cubic centimeter (g/cc) and a water content of approximately 0.67 moles of H.sub.2 O per mole of .alpha.-Ni(OH).sub.2. The interlamellar distance is approximately 8.2 .ANG.. .beta.-Ni(OH).sub.2 is isomorphous with brucite Mg(OH).sub.2 and is a more ordered structure than the .alpha.-phase material. .beta.-Ni(OH).sub.2 has a density of approximately 4.15 g/cc (a lower value of 3.3 g/cc has been reported based on in situ measurements) and a water content of approximately 0.25 moles of H.sub.2 O per mole of .beta.-Ni(OH).sub.2. The interlamellar distance is approximately 4.6 .ANG.. The charged form (oxidized .beta.-Ni(OH).sub.2) of the active material is usually referred to as .beta.-NiOOH indicating a one electron change. .beta.-NiOOH has a structure very similar to .beta.-Ni(OH).sub.2 with a density of 4.68 g/cc (3.8 g/cc in situ) and a water content of 0.22. The interlamellar distance is approximately 4.6 .ANG..
The oxidation of .beta.-Ni(OH).sub.2 to .beta.-NiOOH is a smooth transition resulting in very little disruption of the crystal structure. The state of hydration of the charged and discharged forms is very similar. The change in density amounts to about 12.7% with virtually no expansion of the interlamellar spacing in the lattice. The transition from .beta.-NiOOH to .gamma.-NiOOH, on the other hand, is quite distinct. .gamma.-NiOOH has a density of approximately 3.79 g/cc and a water content of 0.35. This corresponds to a 21% decrease in the material density. The decrease in density corresponds to an increase in the volume of the material. The interlamellar spacing of the brucite layers in .gamma.-NiOOH is approximately 7 .ANG., corresponding to a 49% expansion of the lattice. This expansion is attributed to the incorporation of potassium ions (K.sup.+) and water into the interlamellar spaces.
The relatively large change in density that occurs in the .beta.-NiOOH to .gamma.-NiOOH transition is, to a great extent, responsible for the swelling observed in the electrode. Therefore, an additive which inhibited the formation of .gamma.-NiOOH would serve to reduce swelling in the electrode.
Cadmium has traditionally been used in the nickel electrode as an additive to reduce electrode polarization and swelling. Cadmium has been found to be the most effective additive to prevent swelling in the nickel electrode, particularly when used in conjunction with cobalt. It is believed that cadmium-hydroxide (Cd(OH).sub.2) inhibits the formation of .gamma.-NI(OOH) and therefore reduces electrode swelling. It is further believed that Cd(OH).sub.2 also increases the oxygen overvoltage of the nickel electrode, thus improving charge efficiency.
Cadmium is typically used in the nickel electrode as an additive at the level of less than 5%. Also, when cobalt is used as an additive alone, nickel electrode swelling increases. Also, zinc and magnesium additives are equally effective in preventing swelling but the addition of calcium increases electrode swelling. This is attributed to the large ionic radii of the calcium cation. There is also strong evidence of a quantitative relationship between electrode swelling and the density changes that occur in the active material when charging and discharging. This is most pronounced for the formation of low density .gamma.-phase active material.
Cadmium, however, is environmentally undesirable. Cadmium, as an additive to the nickel electrode, does not have any additional environmental impact in a nickel-cadmium cell because of the presence of a large amount of cadmium in the negative electrode. Any small amount of cadmium in the nickel electrode is relatively insignificant. However, in nickel-zinc, nickel-iron, nickel-metal hydride, nickel-hydrogen or other alkaline rechargeable batteries, even a very small amount of cadmium in the cell has a significant environmental impact. Therefore, a replacement additive was sought which would provide the benefits of cadmium without the undesirable environmental effects. A number of materials have been investigated and several have shown some utility as a performance enhancement additive in the nickel electrode. For example, the effect of cadmium, cobalt, zinc, lead, silver, manganese, cerium, chromium, copper, magnesium, lanthanum, and yttrium has been investigated. Various manganese compounds have been proposed as an additive, as disclosed in U.S. Pat. No. 5,508,121. A considerable amount of work has focused on zinc as the most promising nickel electrode additive as disclosed in U.S. Pat. Nos. 5,549,992; 5,506,070; 5,348,822; 5,523,182; 5,498,403; 5,077,149; 4,844,999; and 4,985,318. This is contrary to early work, associated with the nickel-zinc battery, which postulated that zinc acted as a poison to the nickel electrode reducing both the capacity and cycle life. Other work has suggested that binary, e.g., zinc in combination with manganese, and ternary, e.g., zinc, manganese and iron, additive mixtures can further improve the performance of the nickel-hydroxide electrode. Other nickel electrode additives have been suggested in relation to the overall structure of the active material oriented towards the goal of multiple electron transfer in the nickel electrode reactions.
Concurrent deposition of zinc-hydroxide in a thin film of nickel-hydroxide on a platinum electrode has been investigated by cyclic voltammetry. Theoretical study on thin film materials has not addressed any issues regarding the fabrication of practical battery electrodes. Nickel and cobalt hydroxides have been chemically deposited as a very thin film on a platinum wire in order to study the fundamental properties of the materials. Electrochemical deposition has not been addressed. Prior proposed use of up to 20% zinc-hydroxide by weight is impractical from a battery application viewpoint. It is believed that zinc both inhibits the formation of .gamma.-NiOOH and raises the potential of the oxygen evolution reaction, improving the charge efficiency of the electrode and reducing electrode swelling.
U.S. Pat. No. 4,399,005, describes a process for the concurrent electrochemical deposition of zinc and nickel-hydroxide into the nickel electrode. This concept was developed for the alcohol-based impregnation process. The patent points out that zinc is commonly used as an additive to the pasted-type nickel electrode to improve dimensional stability. The patent goes on to describe a process whereby zinc can be deposited concurrently with nickel-hydroxide by an electrochemical process in which the zinc-nitrate is continuously added to the electrochemical bath during the process.
The nickel electrode is often the performance and life limiting component in a sealed, electrolyte-starved alkaline rechargeable battery. The primary long-term failure mechanism is nickel electrode swelling which results in dry-out of the separators in the electrode stack. As the nickel electrode expands in thickness, its internal volume and porosity increase. This nickel electrode expansion is a macroscopic manifestation of the density and volume changes that occur at the atomic level within the lattice structure of the active material. The microporous structure of the nickel electrode wicks electrolyte from the separator through capillary action and mass transport, causing the separator to contain less electrolyte. This increases the internal impedance of the cell, reducing cell capacity. The loss in cell capacity results in downward spiral in cell performance until failure is reached. This situation is particularly acute in sealed cells which operate in an electrolyte-starved condition for gas management purposes. The situation is also compounded by flat-plate electrode stack construction such as is used in prismatic cells or cylindrical nickel-hydrogen cells. This situation is further compounded in the nickel hydrogen battery cell because the electrode stack is suspended within the pressure vessel cell container such that there is no physical contact between the electrode stack and the cell container wall. Electrolyte lost from the electrode stack is not easily reclaimed. The three-phase reaction of hydrogen on the catalytic anode during discharge requires that the nickel-hydrogen cell operate in an extremely electrolyte-starved condition. This makes this type of cell even more susceptible to swelling of the nickel electrode and the resulting electrode stack dry-out.